In spite of numerous efforts directed toward the study of phenylketonuria (PKU) over four decades, the exact mechanism whereby the enzymatic defect in metabolism of phenylalanine leads to abnormal development of the central nervous system remains obscure. Once the mechanism is understood, therapeutic attempts could be directed toward correcting the basic defect. At present, a restriction of phenylalanine content of the diet, thereby reducing concentration of phenylalanine and/or its metabolites in blood and tissues, including the brain, is the only empirical treatment used. Treatment of children with phenylketonuria by use of phenylalanine restriction in the diet was initiated in this country in about 1955. However, there have been no significant improvements in this highly restrictive treatment since its inception.
It has heretofore been reported that high concentrations of phenylalanine, either administered chronically in the diet or acutely by parental injection, produced significant reductions of cerebral concentrations of a number of proteins (Boggs et al, Fed. Proc., 23, 146, 1964). Other reports have suggested that no depletions in such amino acids occur under similar conditions (Carver, J. Neurochem., 12, 45, 1965). More recently it has been confirmed that a significant depletion of cerebral pools of amino acids occurred by injection or feeding with phenylalanine. However, the pattern of cerebral amino acid depletion in experimental animals takes place in the presence of increased levels of serum amino acids. This phenomenon seems at variance with the traditional concept that cerebral amino acid levels are regulated by internal "blood-brain barriers" of a structural nature which are present in the adult, but poorly developed in the immature brain. Other literature publications in this area include McKean et al, J. of Neurochemistry, 15, pp 235-241, 1968 and Anderson et al, Arch. Neurol., 33, pp 634-636, October, 1976. It has been suggested that perhaps some method may be devised whereby phenylalanine could be excluded from the brain and the cerebral deficiencies of amino acids could also be corrected.
It is apparent that the present empirical treatment for the restriction of phenylalanine in diet is not completely satisfactory. Moreover, continuance of the treatment diet becomes increasingly difficult as the patient grows older. Phenylalanine requirements steadily decrease while the desire for a more normal life, particularly regarding food habits, becomes urgent as the treatment is successful in producing normal physical and intellectual development. Therefore, there is a need for new methods in the treatment of phenylketonuria to control levels of phenylalanine and to inhibit its deleterious effects on the central nervous system.